1. Field of the Invention
The present invention relates generally to improvements in superfluorescent laser light sources, and more particularly pertains to the reduction of excess photon noise in superfluorescent or short coherence laser light sources.
2. Description of Related Art
In the field of laser radiation, it has become known that light from an incoherent source exhibits random fluctuations in signal intensity which are caused by beats between the various randomly phased fourier components making up the linewidth of the source. These fluctuations are known as excess photon noise or wave interaction noise. Every laser source has these two noise components, shot noise and excess photon noise.
Such devices as fiber optic gyroscopes or optical time delay reflectometers (OTDR), which use short coherence light sources such as broadband fiber light sources or superluminescent diodes, for example, suffer from low signal to noise ratios. Increasing the light intensity does not necessarily increase the signal to noise ratio because excess photon noise is proportional to the square of the source intensity, while the shot noise is simply proportional to the average intensity.
The prior art has attempted to improve the signal to noise ratio of laser sources by various techniques. One technique described in Optics Letters, Vol. 16, No. 3, by R. P. Moeller and W. K. Burns, "1.06 .mu.m all-fiber gyroscope with noise subtractor," p. 1902, Dec. 1, 1991, utilizes an optical and electronic configuration for noise subtraction. This method utilizes a plethora of equipment, including a plurality of fiber delay coils, detectors, amplifiers, multipliers, subtractors and polarizers. Another approach described in "Excess Noise Reduction in Fiber Gyroscope Using Broader Spectrum Rinwidth Er-Doped Superfluorescent Fiber Laser," by K. Iwatsuki, p. 281, March 1991, IEEE Photonics Technology Letters, Vol. 3. No. 3, attempts to reduce excess noise by utilizing rare earth-doped superfluorescent laser material. Other approaches, such as found in U.S. Pat. No. 5,311,603 for Highly Efficient Superfluorescent and Fiber Laser/Amplifier for Interferometric Sensors issued May 10, 1994, to Bernard G. Fidric, the present inventor, are directed to improving the light source itself.
Although the problem of reducing excess noise in a laser light source has been known for many years, with a variety of solutions being proposed, none suggest the solution of the present invention, which provides a more simple, direct and more efficient method and apparatus for reducing excess noise in a broadband laser light source.